JavaScript IntelliSense in Visual Studio "Orcas" March CTP - WOW That's awesome!!!
The guys from VS Web tools team did a great job and employ xml comments to make JS Intellisense possible.
Take a look at the screenshots below
ASP.NET AJAX Concepts in Completion List
IntelliSense from Script Libraries for ASPX Pages
There are much more...
Did I get your interest? See the full post at Web Development Tools Team blog.